A “StingRay II,” made by the Harris Corp., can redirect cellphone calls away from cell tower antennae and capture their identifying data and location. Police use them to find people. Some argue that that’s an invasion of privacy. (Courtesy Harris Corp.)

A device that tricks cellphones into sending it their location information and has been used quietly by police and federal agents for years, requires a search warrant before it is turned on, an appeals court in Washington ruled Thursday. It is the fourth such ruling by either a state appeals court or federal district court, and may end up deciding the issue unless the government takes the case to the U.S. Supreme Court or persuades the city’s highest court to reverse the ruling.

The case against Prince Jones in 2013 involved D.C. police use of a “StingRay” cell-site simulator, which enables law enforcement to pinpoint the location of a cellphone more precisely than a phone company can when triangulating a signal between cell towers or using a phone’s GPS function. Civil liberties advocates say the StingRay, by providing someone’s location to police without court approval, is a violation of an individual’s Fourth Amendment right not to be unreasonably searched. The D.C. Court of Appeals agreed in a 2 to 1 ruling, echoing similar rulings in the Maryland Court of Special Appeals and federal district courts in New York City and San Francisco.

“This opinion,” said Nathan F. Wessler of the American Civil Liberties Union, who helped argue the case with the D.C. Public Defender Service, “joins the growing chorus of courts holding that the Fourth Amendment protects against warrantless use of invasive, covert technology to track people’s phones. … We applaud today’s opinion for erecting sensible and strong protections against the government violating people’s privacy in the digital age.”

The U.S. attorney’s office in Washington declined to comment on the ruling. The prosecutors could ask for a rehearing by the three judge panel or the entire appeals court, and if those are denied take the case to the Supreme Court, though Wessler noted that the high court might not be inclined to take a case where there is no dispute among the lower court rulings.

The Justice Department issued policy guidance to its agencies in 2015 that a search warrant must be obtained for all StingRay uses, and though that is not binding on state and local police, the Metropolitan Police Department has said it would abide by that rule. The ACLU has counted 72 cell-site simulators in use in 24 states and the District, but believes there could be many more. Both D.C. and Baltimore police had signed an agreement with the FBI not to disclose or discuss their StingRay device publicly, court records show, and an FBI agent sat with prosecutors during Jones’s trial to advise them on how to handle questions about the device.

The ruling by the D.C. Court of Appeals resulted in all the evidence in the case against Jones being thrown out, and a nine-count felony conviction for sexual abuse, kidnapping, armed robbery and threats being vacated.

Jones was arrested after he allegedly assaulted and robbed two women in separate incidents, after arranging to meet with them through Backpage.com for sexual liaisons. In both cases, the perpetrator took the victims’ cellphones.

After the second incident, D.C. police compared the call records of the victims and found that the same phone number had been used to arrange both meetings. The police then obtained the mobile identification number for the man’s phone, as well as the identification numbers for the victims’ phones, and with the help of the phone companies obtained a general location for the phones, which police said appeared to be traveling together.

Once in the vicinity of the phones, the police turned on the StingRay, court records show, and punched in the identification number (different from the phone number) of the assailant’s phone. The StingRay acts like a cell site antenna, and convinces cellphones to connect to it instead of a real cell site, providing the phone numbers and locations of the phones that connect. The phones are useless during this time because they aren’t connected to an actual network, only the StingRay.

Before long, the assailant’s prepaid cellphone was found on Jones, sitting in a parked car on Minnesota Avenue in Northeast Washington, as were the phones stolen from the victims, police said. The appeals court ruled, and the defense agreed, that if the police had used the StingRay on one of the victims’ phones, instead of Jones’s phone, the search would have been legal because the victims consented to the search.

The judge in Jones’s trial declined to suppress the phone seizure, which in turn led to the knife apparently used in the robberies, the discovery of the victims’ phones and incriminating statements made by Jones and his girlfriend. But the ruling written by Associate Judge Corinne A. Beckwith, joined by Senior Judge Michael W. Farrell, threw out all of that evidence as “fruit of the poisonous tree,” namely the StingRay.

“Locating and tracking a cell-site simulator,” Beckwith wrote, “has the substantial potential to expose the owner’s intimate personal information,” particularly their movements and whereabouts. “A cell-site simulator allows police officers who possess a person’s telephone number to discover that person’s precise location remotely and at will.”

For that reason, Beckwith said, “the use of a cell-site simulator to locate Mr. Jones’s phone invaded a reasonable expectation of privacy and was thus a search.”

Prosecutors argued that everyone knows that the location of a cellphone can be tracked, and at oral argument one noted that every fleeing criminal on television dramas throws away or destroys their phone. Beckwith disregarded that approach, saying that “a person does not lose a reasonable expectation of privacy merely because he or she is made aware of the government’s capacity to invade his or her privacy.”

Associate Judge Phyllis D. Thompson dissented, though she wrote that under ordinary circumstances, she agreed that the government’s use of a StingRay “likely violates the legitimate expectation of privacy.” But Thompson said Jones forfeited that privacy when he drove around with the victims’ stolen cellphones. Beckwith responded that Jones had not been charged or convicted of stealing the phones at the time of the search.

The StingRay issue is separate from another cellphone issue pending before the Supreme Court — whether law enforcement must obtain a warrant before obtaining a cellphone’s historical location data from a phone company. Phone companies record which cell towers are used when a call is made, which police often use to demonstrate a person’s whereabouts at the time of a crime. Those records can be obtained with a court order, and a lower standard of proof, rather than a warrant. The ACLU’s Wessler said that Thursday’s ruling was a “recognition that constitutional protections must keep pace with advancing technology, and is an important reminder of what is at stake as the Supreme Court takes up the issue of police requests for historical cellphone location data.”